Zircon ID-TIMS Pb isotope determination method using multiple ion counters with dynamic multi-collection protocol

ABSTRACT

A zircon ID-TIMDS Pb isotope determination method by multiple ion counters with a dynamic multi-collection protocol is provided. Compared with a commonly used multi-ion counter static determination method, the method provided by the present invention completely eliminates influences of gain differences of the different ion counters on determination results of Pb isotopes. Compared with a conventional single-ion counter determination method with five times of peak-jumps, the method provided by the present invention can obtain all of Pb isotope ratios with two times of peak-jumps, which increases the collection efficiency of Pb isotope ion beams and decreases influences of ion beam stability on Pb isotope analysis results. Consequently, compared with a multi-ion counter static method and a single-ion counter peak-jumping method, the method provided by the present invention improves the Pb isotope analysis precision for the single-grain zircon ID-TIMS U—Pb dating method (with a 205Pb tracer), having application potentials.

CROSS REFERENCE OF RELATED APPLICATION

The application claims priority under 35 U.S.C. 119(a-d) to CN201911034352.8, filed Oct. 29, 2019.

BACKGROUND OF THE PRESENT INVENTION Field of Invention

The present invention relates to a technical field of isotope massspectrometric analysis, and more particularly to a Pb isotopedetermination method by multiple ion counters with a dynamicmulti-collection protocol, which can be applied in the single-grainzircon ID-TIMS (Isotope Dilution-Thermal Ionization Mass Spectrometry)U—Pb dating technology.

Description of Related Field

The zircon U—Pb method is a most important isotope geochronology methodand is widely applied in dating of various geological events, such asgranite formation age and stratigraphic age. The zircon U—Pb methodsmainly comprise the SIMS (Secondary Ion Mass Spectrometry) in-situmethod, the LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-MassSpectrometry) in-situ method, and the ID-TIMS (Isotope Dilution-ThermalIonization Mass Spectrometry) method. The ID-TIMS method has thecharacteristic of high precision and is the primary method for zirconU—Pb age determination. In recent years, with the continuous improvementof the analysis technology, better than 0.05% precision of agedetermination has been achieved with the ID-TIMS method.

The ID-TIMS U—Pb method requires to dissolve the zircon grains and add a²⁰⁵Pb-²³⁵U tracer, then separate U and Pb from the sample with a smallanion exchange column, and finally determine the isotope ratios of U andPb with a thermal ionization mass spectrometer and obtain the U—Pb ageof the zircon through calculation. Because only the trace amount of Pb(generally in pg, i.e., 10⁻¹² g level) is contained inside asingle-grain zircon, the Pb isotopes therefore are generally determinedwith the ion counter.

In the conventional single-grain zircon ID-TIMS U—Pb method, the Pbisotopes are generally determined through peak-jumping with the centerchannel SEM (Secondary Electronic Multiplier) or Daly detector; that isto say, through changing the magnetic field, the Pb isotopes including²⁰⁴Pb, ²⁰⁵Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb successively enter the centerchannel SEM or the Daly detector and are determined one by one. Themethod requires five jumps to complete one cycle of zircon Pb isotopedetermination. Thus, the collection efficiency of the Pb isotope ionbeams is low, and consequently the determination time is long. In orderto reach the high precision, the determination time for each samplerequires 3-4 hours. Meanwhile, because the different isotopes aresuccessively determined, the different isotopes are not determined atthe same time, the ion beam stability has influences on the precisionand accuracy of the Pb isotope determination results.

The new thermal ionization mass spectrometer is generally equipped withthe multi-ion counter system. For example, the TRITON PLUS massspectrometer produced by the Thermo Fisher Scientific Company isequipped with multiple ion counters specifically for Pb isotope analysisfor the zircon ID-TIMS U—Pb method; the Phoenix mass spectrometerproduced by Isotopx Company is equipped with the multiple Channeltronion counters. The current common multi-ion counter determination methodadopts the static multi-collection method. The static method can collectand determine all of the Pb isotopes at the same time withoutpeak-jumping. With the static method, the ion beam collection efficiencyis high; the Pb isotope determination results are not influenced by theion beam stability; and the determination time for a sample can berelatively short. However, because gain differences exist between thedifferent ion counters of the multi-ion counter system and the stabilityof the gain of the ion counters is usually poor, before and after thedetermination of a sample, it is necessary to determine a standard suchas NIST981 or NIST982 Pb once with the multi-ion counter static method,so as to correct the gain differences between the multiple ion counters.This is usually named as standard-sample-bracketing method, i.e. SSBmethod. Nevertheless, because the gain stability of the ion counters isrelatively poor, when determining the Pb isotopes with the staticmulti-collection method by the multiple ion counters, the precision andaccuracy of the Pb isotope determination results are still not idealeven with the SSB method. Thus, currently, the multi-ion counter systemis rarely applied in zircon ID-TIMS U—Pb dating.

SUMMARY OF THE PRESENT INVENTION

In view of the above problems, aiming at a Pb isotope analysistechnology for the single-grain zircon ID-TIMS (Isotope Dilution-ThermalIonization Mass Spectrometry) U—Pb dating (with a ²⁰⁵Pb tracer), themain purpose of the present invention is to establish a high-precisionanalysis method for Pb isotopes of a ²⁰⁵Pb—Pb mixture by multiple ioncounters with a dynamic multi-collection protocol.

A zircon ID-TIMS Pb isotope determination method by multiple ioncounters with a dynamic multi-collection protocol is provided, as shownin FIG. 1, comprising steps of:

(S1) adopting at least four ion counters of a thermal ionization massspectrometer, respectively denoted as IC1, IC2, IC3 and IC4;

(S2) designing two times of peak-jumps for isotope determination,respectively denoted as J1 and J2;

(S3) determining all of five isotopes of ²⁰⁴Pb, ²⁰⁵Pb, ²⁰⁶Pb, ²⁰⁷Pb and²⁰⁸Pb, particularly comprising steps of at a first jump (J1),determining ⁰⁵Pb, Pb, ²⁰⁷Pb and ²⁰⁸Pb respectively by the four ioncounters IC1, IC2, IC3 and IC4; at a second jump (J2), determining²⁰⁴Pb, ²⁰⁰⁵Pb, ²⁰⁶Pb and ²⁰⁷Pb respectively by the four ion countersIC1, IC2, IC3 and IC4; and

(S4) through appropriately combining Pb isotope signal intensitiesobtained by the first and second jumps, calculating and obtaining Pbisotope ratios of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁵Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb,particularly comprising steps of:

(S41) with ²⁰⁷Pb determined by the IC3 from the first jump and ²⁶Pbdetermined by the IC3 from the second jump, directly obtaining theisotope ratio of ²⁰⁷Pb/²⁰⁶Pb, denoted as ²⁰⁷Pb_(IC3-J1)/²⁰⁶Pb_(IC3-J2);

(S42) with ²⁰⁸Pb determined by the IC4 from the first jump and ²⁰⁷Pbdetermined by the IC4 from the second jump, obtaining ²⁰⁸Pb/²⁰⁷Pb,denoted as ²⁰⁸Pb_(IC4-J1)/²⁰⁷Pb_(IC4-J2); through calculating with aformula of²⁰⁸Pb/²⁰⁶Pb=²⁰⁷Pb_(IC3-J1)/²⁰⁶Pb_(IC3-J2)×²⁰⁸Pb_(IC4-J1)/²⁰⁷Pb_(IC4-J2),obtaining ²⁰⁸Pb/²⁰⁶Pb;

(S43) with ²⁰⁵Pb determined by the IC2 from the second jump and ²⁰⁶Pbdetermined by the IC2 from the first jump, directly obtaining²⁰⁵Pb/²⁰⁶Pb, denoted as ²⁰⁵Pb_(IC2-J2)/²⁰⁶Pb_(IC2-J1); and

(S44) with ²⁰⁴Pb determined by the IC1 from the second jump and ²⁰⁵Pbdetermined by the IC from the first jump, obtaining ²⁰⁴Pb/²⁰⁵Pb, denotedas ²⁰⁴Pb_(IC1-J2)/²⁰⁵Pb_(IC1-J1); through calculating with a formula of²⁰⁴Pb/²⁰⁶Pb=²⁰⁵Pb_(IC2-J2)/²⁰⁶Pb_(IC2-J1)×²⁰⁴Pb_(IC1-J2)/²⁰⁵Pb_(IC1-J1),obtaining ²⁰⁴Pb/²⁰⁶Pb.

According to the above method, the ratios of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁵Pb/²⁰⁶Pb,²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb are all obtained through isotope signalintensities determined by peak-jumps with a same ion counter, thuscompletely eliminating influences of gain differences of the differention counters on determination results of the Pb isotopes.

According to the method provided by the present invention, a linearinterpolation method is adopted to correct influences of ion beamstability on determination results of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁵Pb/²⁰⁶Pb,²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb during a peak-jump determination processwith the multiple ion counters (newly produced thermal ionization massspectrometers are all installed with a linear interpolation calculationsoftware for the dynamic determination method).

The method provided by the present invention has beneficial effects asfollows.

Compared with the traditional determination method for the Pb isotopesof a ²⁰⁵Pb—Pb mixture by five times of peak-jumps with a single ioncounter (SEM (Secondary Electronic Multiplier) or Daly detector), themethod provided by the present invention can obtain all of the Pbisotope ratios only with two times of peak-jumps, thus increasing thecollection efficiency of the Pb ion beam by 2.5 times and meanwhiledecreasing the influences of the ion beam stability on the Pb isotopedetermination results.

Compared with the Pb isotope determination method by the multiple ioncounters with a static multi-collection protocol, the method provided bythe present invention completely eliminates the influences of the gaindifferences of the different ion counters on the determination resultsof the Pb isotopes, thus greatly improving the determination precisionof the Pb isotopes of a ²⁰⁵Pb—Pb mixture.

The method provided by the present invention is especially applicable tothe Pb isotope analysis for the single-grain zircon ID-TIMS U—Pb datingmethod.

The protocol of the method provided by the present invention has theuniversality and is suitable for the various types of mass spectrometersequipped with the multi-ion counter system which can be applied for thePb isotope analysis of a ²⁰⁵Pb—Pb mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch view to illustrate a dynamic multi-collection methodfor Pb isotopic analysis of a ²⁰⁵Pb—Pb mixture using multiple ioncounters according to the present invention.

FIG. 2 is a configuration diagram of a multi-ion counter system of aTRITON PLUS thermal ionization mass spectrometer according to thepresent invention.

FIG. 3 shows comparison of determination results of ²⁰⁷Pb/²⁰⁶Pb ofNIST981 with a method provided by the present invention, a multi-ioncounter static method, and a single-ion counter peak-jumping method. InFIG. 3, a full line represents a mean, and dotted lines represent arange of mean±2SD.

FIG. 4A and FIG. 4B show determination results of Qinghu zircon U—Pbage, wherein: FIG. 4A is a U—Pb Concordia age diagram; and FIG. 4B is adiagram of weighted average ²⁰⁶Pb/²³⁸U age.

FIG. 5A and FIG. 5B show determination results of TEMORA zircon U—Pbage, wherein: FIG. 5A is a U—Pb Concordia age diagram; and FIG. 5B is adiagram of weighted average ²⁰⁶Pb/²³⁸U age.

In figures: J1 and J2 respectively represent a first jump and a secondjump; IC1, IC2, IC3 and IC4 respectively represent the first, second,third and fourth ion counters; SEM represents a secondary electronicmultiplier; and CDD represents a compact discrete dynode multiplier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is further described in detail with examples asfollows.

Referring to FIG. 1, the present invention provides a determinationmethod for Pb isotopes of a ²⁰⁵Pb—Pb mixture with a dynamicmulti-collection protocol by multiple ion counters of a thermalionization mass spectrometer. The method is suitable for various typesof mass spectrometers equipped with a multi-ion counter system which canbe applied for Pb isotope analysis of a ²⁰⁵Pb—Pb mixture. Theestablished method comprises steps of: adopting four ion counters; withtwo times of peak-jumps, determining all of the Pb isotopes including²⁰⁴Pb, ²⁰⁵Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb, wherein ²⁰⁵Pb is from a tracer;then, through designing algorithms, eliminating the influences of thegain differences of the multiple ion counters and decreasing theinfluences of ion beam stabilities on the Pb isotope determinationresults. The established method is especially applicable tohigh-precision Pb isotope determination for a single-grain zirconID-TIMS (Isotope Dilution-Thermal Ionization Mass Spectrometry) U—Pbmethod (using a ²⁰⁵Pb tracer). With the TRITON PLUS thermal ionizationmass spectrometer produced by the Thermo Fisher Scientific Company as anexample, the present invention is described as follows.

1. Mass Spectrometric Determination Method

The TRITON PLUS thermal ionization mass spectrometer of Thermo FisherScientific Company is equipped with a multi-ion counter (MIC) systemspecifically for Pb isotope analysis for zircon ID-TIMS U—Pb dating (asshown in FIG. 2). The MIC system comprises three SEM (SecondaryElectronic Multiplier) and two CDD (Compact Discrete Dynode) electronicmultipliers, wherein the ion counter IC5 (CDD) is bundled on the L4Faraday cup (a position of the IC5 can be slightly adjusted throughadjusting the position of the L4 Faraday cup); the ion counters IC2(SEM) and IC4 (CDD) are located on the L5 Faraday cup position.According to the present invention, three SEM ion counters and one CDDion counter therein are used with a dynamic multi-collectionpeak-jumping method to determine the Pb isotopes of a ²⁰⁵Pb—Pb mixture.For the multiple ion counters of TRITON PLUS, the detailed settings ofthe multiple ion counters and the peak-jumping method thereof are listedin Table 1. Through designing two times of peak-jumps, the ion beams ofdifferent Pb isotopes are collected, wherein: at the first jump, thevirtual mass referring to the center of the multi-collection system ofthe instrument is set to be 223.04, the ion counter IC5-L4 (CDD)collects ²⁰⁸Pb, IC1 B (SEM) collects ²⁰⁷Pb, IC2-L5 (SEM) collects ²⁰⁶Pb,and IC3A (SEM) collects ²⁰⁵Pb; at the second jump, the virtual massreferring to the center of the multi-collection system of the instrumentis set to be 221.95, the ion counter IC5-L4 (CDD) collects ²⁰⁷Pb, IC1 B(SEM) collects ²⁰⁶Pb, IC2-L5 (SEM) collects ²⁰⁵Pb, and IC3A (SEM)collects ²⁰⁴Pb. For each jump, the integration time is 4.194 seconds,and the idle time before integrations is set to 1 second. Because onlythe position of IC5-L4 can be slightly adjusted through adjusting theposition of the L4 Faraday cup and the positions of IC2-L5, IC3A andIC1-B cannot be adjusted, the present invention ensures perfect peakalignments of different ion counters for the two jumps through settingZoom dispersion parameters and adjusting the position of the L4 Faradaycup.

When using the multi-ion counter system, it is required to determine thedead time and the yield of each ion counter. The present inventionfirstly determines the dead time of each ion counter by measuring the²⁰⁸Pb/²⁰⁶Pb of NIST981 Pb standard using a peak-jumping method with anion counter; that is to say, for each ion counter, the ²⁰⁸Pb signalintensity is increased from 1 mV to 10 mV stepwise, and under differentsignal intensities, the ratios of ²⁰⁸Pb/²⁰⁶Pb are respectivelydetermined by a peak-jumping method; through monitoring the correlationof the determination results of the ratio of ²⁰⁸Pb/²⁰⁶Pb with the ²⁰⁸Pbsignal intensity, the dead time of each ion counter is determined.Moreover, the yield of the ion counters was determined by switching astable 5-10 mV ²⁰⁸Pb ion beam sequentially into the center Faraday cup,IC5, IC1B, IC2 and IC3A as shown in Table 2; that is to say, the ²⁰⁸Pbsignal is adjusted to be stable at 5-10 mV, and the ²⁰⁸Pb signal isdetermined successively with the center Faraday cup and the differention counters one by one; through the ratios of the ²⁰⁸Pb measured byeach ion counter to that determined by the center Faraday cup, the yieldof each ion counter relative to the center Faraday cup is obtained. Itis required to ensure that the yield of each ion counter is greater than90%; if the yield for an ion counter is smaller than 90%, the highvoltage applied for the ion counter should be appropriately increased.

TABLE 1 Dynamic multi-collection method for Pb isotopic determination ofa ²⁰⁵Pb—Pb mixture by multiple ion counters of TRITON PLUS thermalionization mass spectrometer Virtual mass of center Integration IdleFaraday IC4-L5 IC3-A IC2-L5 RPQ/IC1-B IC5-L4 time time cup (CDD) (SEM)(SEM) (SEM) (CDD) (s) (s) J1 223.04 ²⁰⁴Pb ²⁰⁵Pb ²⁰⁶Pb ²⁰⁷Pb ²⁰⁸Pb 4.1941 J2 221.95 ²⁰³Tl ²⁰⁴Pb ²⁰⁵Pb ²⁰⁶Pb ²⁰⁷Pb 4.194 1

TABLE 2 Method for Yield determination of the multiple ion countersCenter Integration Idle RPQ/ Faraday time time Line IC3 A IC2-L5 IC1 BIC5-L4 cup (s) (s) 1 ²⁰⁸Pb 4.194 3 2 ²⁰⁵Pb ²⁰⁶Pb ²⁰⁷Pb ²⁰⁸Pb 222.934.194 3 3 ²⁰⁷Pb ²⁰⁸Pb 224.01 4.194 3 4 ²⁰⁸Pb 225.08 4.194 3 5 ²⁰⁸Pb226.13 4.194 3

2. Sample Determination Process

When determining the Pb isotopes, firstly slowly increasing atemperature of the filament to about 1000° C.; tuning and focusing theion beam with the ²⁰⁸Pb signal detected by the IC5-L4 or with the ²⁰⁶Pbsignal detected by the IC2-L5; subsequently, slowly ramping thetemperature of the filament to increase the ion beam intensity; afterthe ion beam intensity reaches the expected value, starting to collectthe data. For each data block, 25 cycles of data are collected; andthere are totally 20 blocks of data to be acquired. Prior to each bock,peak centering is run for the first and the second jumps with the ²⁰⁶Pbsignal detected by IC2-L5 and IC1-B, respectively, to re-locate the ionbeams into the corresponding ion counters; every four blocks, the ionbeam is re-focused with the ²⁰⁶Pb signal detected by IC2-L5 once.

For the zircon sample, after completing the determination of the Pbisotopes, further increasing the temperature of the filament to about1200° C.-1300° C.; and determining the U isotope composition(determining UO₂ ⁺). The U isotopes are determined by peak-jumping usingthe center channel SEM (IC1 C). With ¹⁷O/¹⁶O=0.00039 and¹⁸O/¹⁶O=0.00205, the interferences of ²³⁵U¹⁷O¹⁸O on ²³⁸U¹⁶O₂ arecorrected. The fractionation effects of the U isotopes are correctedthrough an external calibration method with the U 500 determinationresults. The detailed U mass spectrometric determination method refersto document 1.

-   Reference document 1: Chu et al., Ultra-low blank analytical    procedure for high precision CA-ID-TIMS U—Pb dating of single grain    zircons, Chinese Science Bulletin, 2016, Volume 61, pages 1121-1129.

3. Method for Processing Pb Isotope Data

The method comprises steps of:

(1) with ²⁰⁷Pb determined from the first jump and ²⁰⁶Pb determined fromthe second jump by the IC1-B (SEM), obtaining the ratio of ²⁰⁷Pb/²⁰⁶Pb;with a linear interpolation method, correcting the influences of ionbeam stability on the determination results of ²⁰⁷Pb/²⁰⁶Pb;

(2) with ²⁰⁸Pb determined from the first jump and ²⁰⁷Pb determined fromthe second jump by the IC5 (CDD), obtaining the ratio of ²⁰⁸Pb/²⁰⁷Pb;with the linear interpolation method, correcting the influences of theion beam stability on the determination results of ²⁰⁸Pb/²⁰⁷Pb;

(3) calculating with a formula of ²⁰⁸Pb/²⁰⁶Pb=²⁰⁸Pb/²⁰⁷Pb×²⁰⁷Pb/²⁰⁶Pb,obtaining ²⁰⁸Pb/²⁰⁶Pb;

(4) with ²⁰⁵Pb determined from the second jump and ²⁰⁶Pb determined fromthe first jump by the IC2-L5 (SEM), obtaining the ratio of ²⁰⁵Pb/²⁰⁶Pb;with the linear interpolation method, correcting the influences of theion beam stability on the determination results of ²⁰⁵Pb/²⁰⁶Pb;

(5) with ²⁰⁴Pb determined from the second jump and ²⁰⁵Pb determined fromthe first jump by the IC3 (SEM), obtaining the ratio of ²⁰⁴Pb/²⁰⁵Pb;with the linear interpolation method, correcting the influences of theion beam stability on the determination results of ²⁰⁴Pb/²⁰⁵Pb; and

(6) calculating with a formula of ²⁰⁴Pb/²⁰⁶Pb=²⁰⁵Pb/²⁰⁶Pb×²⁰⁴Pb/²⁰⁵Pb,obtaining ²⁰⁴Pb/²⁰⁶Pb.

Through the above method, all of the Pb isotope ratios after correctingthe influences of the ion beam stabilities through the linearinterpolation method are obtained, including ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁵Pb/²⁰⁶Pb,²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb.

The ratios of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁵Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pbobtained through the above method are all equivalently determined by asame ion counter using a peak-jumping method, thus completelyeliminating influences of gain differences of different ion counters ondetermination results of the Pb isotopes.

According to the present invention, the isotope signal data collected bythe IC4-L5 ion counter is not used.

During the isotope determination process of ultra-small amount of Pb (pglevel), the trace amount of impurities in the sample, such as theorganics, may influence the determination result of the Pb isotopes.Thus, the impurities require to be gradually burned off; and because theinfluences of the interfering substances may exist, the data at thebeginning stage of a determination are generally required to bediscarded. During the data processing process, these abnormal data aredeleted with the Tripoli software (can be downloaded fromhttp://www.earth-time.org/), and then the final Pb isotope determinationresults can be obtained.

The fractionation effects of the Pb isotopes for the zircon sample arerequired to be corrected through an external calibration method with theNIST981 Pb isotope determination results.

Example 1: Pb Isotope Analysis of ²⁰⁵Pb-NIST981 Mixed Solution

The present invention firstly conducts the Pb isotope determination on amixed solution of a NIST981 standard solution and a ²⁰⁵Pb tracer, so asto evaluate the precision and accuracy of the determination method. ThePb isotope determination results obtained through the multi-ion counterdynamic multi-collection method provided by the present invention arecompared with the Pb isotope determination results obtained throughother methods, including the multi-ion counter static multi-collectionmethod and the single-ion counter peak-jumping method.

The ²⁰⁵Pb-NIST981 mixed solution is prepared through following steps of:taking 500 μL of ²⁰⁵Pb-²³⁵U tracer (the concentration of ²⁰⁵Pb is 9.223pmol/g; the abundances of ²⁰⁴Pb, ²⁰⁵Pb, ²⁰⁶Pb, Pb and ²⁰⁸Pb arerespectively 0.0046%, 99.85%, 0.0384%, 0.0308% and 0.0731%) and 50 μL of290 ng/g NBS981 standard solution; adding into a 3 mL Teflon PFA beaker,and capping the beaker tightly; placing the beaker onto a hot-plate, andfluxing at 80° C. for at least one week, so as to ensure thesample-tracer Pb isotopic equilibration. 2 μL of the ²⁰⁵Pb-NIST981 mixedsolution (corresponding to 50 pg of Pb) are loaded on zone-fined highpurity Re filaments for mass spectrometric determination, and thedetailed sample loading method refers to document 1. Preparation of the²⁰⁵Pb-NIST981 mixed solution and the sample loading are conducted in aclass 100 fume hood and/or class 100 clean bench in a class 1000 cleanroom.

After subtraction of the contribution of the tracer from the Pb isotopedetermination results of the mixed solution, the determination resultsof ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb of the NIST981 are obtained,listed in Table 3.

Correspondingly, the mean (n=20) of the determination results of²⁰⁴Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb of the NIST981 Pb standard(also with a sample load amount of 50 pg) by the multiple ion countersof the TRITON PLUS thermal ionization mass spectrometer with a staticcollection method (the IC4-L5 CDD collects ²⁰⁴Pb, the IC2-L5 SEMcollects ²⁰⁶Pb, the RPQ/IC1B SEM collects ²⁰⁷Pb, and the IC5-L4 CDDcollects ²⁰⁸Pb) are also listed in Table 3 (determining 30 blocks foreach sample, and collecting 20 cycles of data for each block). The mean(n=20) of the determination results of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and²⁰⁸Pb/²⁰⁶Pb of the NIST981 Pb standard (again with a sample load amountof 50 pg) through a peak-jumping method (totally four jumps, forrespectively determining ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb) with the centerchannel SEM ion counter of the TRITON PLUS thermal ionization massspectrometer are also listed in Table 3 (determining 15 blocks for eachsample, and collecting 20 cycles of data for each block).

FIG. 3 shows comparison of ²⁰⁷Pb/²⁰⁶Pb determination results of NIST981with above three determination methods.

It can be seen from Table 3 and FIG. 3 that: the determination internalprecision and external precision of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and²⁰⁸Pb/²⁰⁶Pb of the NIST981 Pb standard with the method provided by thepresent invention are obviously better than that with the single-ioncounter peak-jumping method and the multi-ion counter static method.

TABLE 3 Pb isotope determination results of NIST981 with multi-ioncounter dynamic multi-collection method 2RSE 2RSE 2RSE Determinationtimes ²⁰⁴Pb/²⁰⁶Pb (%) ²⁰⁷Pb/²⁰⁶Pb (%) ²⁰⁸Pb/²⁰⁶Pb (%) 1 0.05895 0.0800.91367 0.045 2.1635 0.036 2 0.05898 0.043 0.91393 0.028 2.1592 0.024 30.05898 0.037 0.91243 0.023 2.1597 0.019 4 0.05896 0.038 0.91287 0.0202.1573 0.022 5 0.05884 0.050 0.91246 0.021 2.1609 0.026 6 0.05918 0.0840.91376 0.027 2.1603 0.040 7 0.05892 0.062 0.91387 0.043 2.1624 0.035 80.05901 0.091 0.91339 0.032 2.1599 0.038 9 0.05908 0.081 0.91409 0.0512.1609 0.034 10 0.05912 0.079 0.91390 0.025 2.1615 0.030 11 0.05913 0.120.91320 0.025 2.1608 0.046 12 0.05912 0.059 0.91284 0.026 2.1603 0.02713 0.05920 0.062 0.91320 0.032 2.1617 0.025 14 0.05915 0.070 0.913620.033 2.1586 0.032 15 0.05917 0.067 0.91391 0.025 2.1598 0.034 160.05912 0.066 0.91298 0.026 2.1629 0.028 17 0.05924 0.063 0.91411 0.0312.1579 0.031 18 0.05908 0.072 0.91349 0.027 2.1618 0.033 19 0.059100.068 0.91413 0.029 2.1592 0.026 20 0.05915 0.048 0.91375 0.034 2.15880.029 Mean ± SD 0.05907 ± 0.00011 0.91348 ± 0.00054 2.1604 ± 0.0016IC1-C peak-jumping 0.05909 ± 0.00021 0.91323 ± 0.00071 2.1610 ± 0.0023method: Mean ± SD (n = 20) MIC static method: 0.05830 ± 0.00028 0.9128 ±0.0014 2.1680 ± 0.0032 Mean ± SD (n = 20) *RSE: relative standard error;SD: standard deviation

Example 2: ID-TIMS U—Pb Age Determination for Qinghu Standard Zircon

The Qinghu zircon is a calibration standard used for SIMS (Secondary IonMass Spectrometry) and LA-ICP-MS (Laser Ablation-Inductively CoupledPlasma-Mass Spectrometry) zircon U—Pb age determination. Researchershave previously conducted U—Pb age determination for the Qinghu standardzircon with the ID-TIMS method, and the obtained ²⁰⁶Pb/²³⁸U weightedaverage age is 159.45±0.16Ma (±2SE) (referring to the document 2).Recently, the ID-TIMS U—Pb laboratory of Massachusetts Institute ofTechnology has also conducted ID-TIMS age determination for the Qinghuzircon, and the ²⁰⁶Pb/²³⁸U weighted average age is 159.36±0.06 Ma(±2SE).

The present invention conducts ID-TIMS U—Pb age determination on theQinghu standard zircon with a ²⁰⁵Pb-²³⁵U tracer, and the sampledigestion and chemical separation method refers to the document 1.During the mass spectrometric determination, the Pb isotopes aredetermined with the multi-ion counter dynamic method provided by thepresent invention, and U is determined with the center channel SEM usinga peak-jumping method. The ²⁰⁶Pb/²⁰⁴Pb determination results are between360-2560, and the age determination results are illustrated in FIG. 4Aand FIG. 4B. The ²⁰⁶Pb/²³⁸U weighted average age is 159.51±0.13 Ma(±2SE) (2RSE=0.08%, n=8; RSE represents relative standard error, thesame hereafter), which is consistent with the values reported in thedocument 2 and the determination results by the ID-TIMS U—Pb laboratoryof Massachusetts Institute of Technology within analytical errors. It isindicated that: with the multi-ion counter dynamic Pb isotope analysismethod provided by the present invention, accurate U—Pb agedetermination results of a zircon sample can be obtained.

-   Reference document 2: Li X H, Liu Y, Li Q L, et al., Precise    determination of Phanerozoic zircon Pb/Pb age by multi-collector    SIMS without external standardization, Geochemistry Geophysics    Geosystems, 2009, 10: Q04010.

Example 3: ID-TIMS U—Pb Age Determination for TEMORA Standard Zircon

The TEMORA zircon is an international reference standard zircon for U—Pbgeochronology. Researchers have previously conducted U—Pb agedetermination on the TEMORA zircon with the ID-TIMS method, and theobtained ²⁰⁶Pb/²³⁸U weighted average age is 416.78±0.33 Ma (±2SE)(referring to the document 3). Recently, the inventors have alsoconducted ID-TIMS age determination on the TEMORA zircon at the ID-TIMSU—Pb laboratory of Massachusetts Institute of Technology, and the²⁰⁶Pb/²³⁸U weighted average age is 417.71±0.12 Ma (±2SE).

The present invention conducts ID-TIMS U—Pb age determination on theTEMORA standard zircon with a ²⁰⁵Pb-²³⁵U tracer, similarly as theexample 2, and the sample digestion and chemical separation methodrefers to the document 1. During the mass spectrometric determination,the Pb isotopes are determined with the multi-ion counter dynamic methodprovided by the present invention, and U is determined with the centerchannel SEM using a peak-jumping method. The ²⁰⁶Pb/²⁰⁴Pb determinationresults are between 300-2630, and the age determination results areillustrated in FIG. 5A and FIG. 5B. The ²⁰⁶Pb/²³⁸U weighted average ageis 418.49±0.31Ma (±2SE) (2RSE=0.07%, n=7); although it is relativelyhigher than the values (²⁰⁶Pb/²³⁸U age=416.78±0.33 Ma) reported in thedocument 3, it is consistent with the determination results (²⁰⁶Pb/²³⁸Uage=417.71±0.12 Ma) of the same batch of TEMORA standard zircon by theinventors at the ID-TIMS U—Pb laboratory of Massachusetts Institute ofTechnology within analytical errors. The example 3 further indicatesthat: with the dynamic multi-ion counter Pb isotope analysis methodprovided by the present invention, accurate zircon U—Pb agedetermination results can be obtained.

-   Reference document 3: Black L P, Kamo S L, Allen C M, et al., TEMORA    1: A new zircon standard for Phanerozoic U—Pb geochronology, Chem.    Geol., 2003, 200: 155-170.

Compared with the conventional single-ion counter peak-jumping method,the multi-ion counter dynamic collection method provided by the presentinvention increases the collection efficiency of the Pb isotope ionbeams by 2.5 times and meanwhile decreases the influences of the ionbeam stability on the analysis results of the Pb isotopes. Thus,time-normalized precision for Pb isotope determination for thesingle-grain zircon ID-TIMS U—Pb analysis is improved, and thus the massspectrometric determination time of the Pb isotopes can be shortened. Inorder to obtain a high-precision single-grain zircon ID-TIMS U—Pb age(better than 0.1%), the conventional single-ion counter peak-jumpingmethod generally requires 3.5 hours to complete the Pb isotope analysisof one zircon U—Pb dating sample (including the impurity burn-off time),while the method provided by the present invention generally requiresonly 2 hours (also including the impurity burn-off time).

It should be understood that: for one of ordinary technique in thefield, improvements and variations can be made based on the abovedescription, which should be all encompassed in the protection scope ofthe claims of the present invention.

What is claimed is:
 1. A zircon ID-TIMS (Isotope Dilution-ThermalIonization Mass Spectrometry) Pb Isotope determination method usingmultiple ion counters with a dynamic multi-collection protocol,comprising steps of: (S1) for a Pb sample in which a ²⁰⁵Pb tracer isadded, adopting at least four ion counters of a thermal ionization massspectrometer, respectively denoted as IC1, IC2, IC3 and IC4; (S2)designing two times of peak-jumps for isotope determination,respectively denoted as J1 and J2; (S3) determining all of five isotopesof ²⁰⁴Pb, ²⁰⁵Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb, particularly comprising stepsof: at a first jump (J1), determining ²⁰⁵Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pbrespectively by the four ion counters IC1, IC2, IC3 and IC4; at a secondjump (J2), determining ²⁰⁴Pb, ²⁰⁵Pb, ²⁰⁶Pb and ²⁰⁷Pb respectively by thefour ion counters IC1, IC2, IC3 and IC4; and (S4) through appropriatelycombining Pb isotope signal intensities obtained by the first and secondjumps, calculating and obtaining Pb isotope ratios of ²⁰⁴Pb/²⁰⁶Pb,²⁰⁵Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb, particularly comprising stepsof: (S41) with ²⁰⁷Pb determined by the IC3 from the first jump and ²⁰⁶Pbdetermined by the IC3 from the second jump, obtaining ²⁰⁷Pb/²⁰⁶Pb,denoted as ²⁰⁷Pb_(IC3-J1)/²⁰⁶Pb_(IC3-J2); (S42) with ²⁰⁸Pb determined bythe IC4 from the first jump and ²⁰⁷Pb determined by the IC4 from thesecond jump, obtaining ²⁰⁸Pb/²⁰⁷Pb, denoted as²⁰⁸Pb_(IC4-J1)/²⁰⁷Pb_(IC4-J2); through calculating with a formula of²⁰⁸Pb/²⁰⁶Pb=²⁰⁷Pb_(IC3-J1)/²⁰⁶Pb_(IC3-J2)×²⁰⁸Pb_(IC4-J1)/²⁰⁷Pb_(IC4-J2),obtaining ²⁰⁸Pb/²⁰⁶Pb; (S43) with ²⁰⁵Pb determined by the IC2 from thesecond jump and ²⁰⁶Pb determined by the IC2 from the first jump,obtaining ²⁰⁵Pb/²⁰⁶Pb, denoted as ²⁰⁵Pb_(IC2-J2)/²⁰⁶Pb_(IC2-J1); and(S44) with ²⁰⁴Pb determined by the IC1 from the second jump and ²⁰⁵Pbdetermined by the IC1 from the first jump, obtaining ²⁰⁴Pb/²⁰⁵Pb,denoted as ²⁰⁴Pb_(IC1-J2)/²⁰⁵Pb_(IC1-J1); through calculating with aformula of²⁰⁴Pb/²⁰⁶Pb=²⁰¹Pb_(IC2-J2)/²⁰⁶Pb_(IC2-J1)×²⁰⁴Pb_(IC1-J2)/²⁰⁵Pb_(IC1-J1),obtaining ²⁰⁴Pb/²⁰⁶Pb.
 2. The determination method, as recited in claim1, wherein: the obtained ratios of ²⁰⁴Pb/²⁰⁶Pb, ²⁰⁵Pb/²⁰⁶Pb, ²⁰⁷Pb/²⁰⁶Pband ²⁰⁸Pb/²⁰⁶Pb are all determined by peak-jumps with a same ioncounter, which completely eliminates influences of gain differences ofthe ion counters on determination results of the Pb isotopes.
 3. Thedetermination method, as recited in claim 1, wherein: in the steps ofS41-S44, a linear interpolation method is adopted to correct influencesof ion beam stability on determination results of each isotope ratioduring a peak-jump determination process by the multiple ion counters.4. The determination method, as recited in claim 1, wherein: whendetermining Pb isotope ratios of a sample by the determination method,because ²⁰⁴Pb/²⁰⁶Pb is obtained through a calculation of²⁰⁴Pb/²⁰⁶Pb=²⁰⁵Pb_(IC2-J2)/²⁰⁶Pb_(IC2-J1)×²⁰⁴Pb_(IC1-J2)/²⁰⁵Pb_(IC1-J1),the method is only applicable to Pb isotope analysis of a ²⁰⁵Pb—Pbmixture.
 5. The determination method, as recited in claim 1, wherein:the method completes determination of all Pb isotope ratios with onlytwo times of peak-jumps; compared with a conventional single-ion counter(SEM (Secondary Electronic Multiplier) or Daly detector) method whichcompletes determination of all Pb isotopes with five times ofpeak-jumps, the determination method increases a collection efficiencyof Pb ion beams by 2.5 times and meanwhile decreases influences of ionbeam stability during a Pb isotope determination process ondetermination results of each isotope ratio, so that time-normalizedprecision for Pb isotope determination of a ²⁰⁵Pb—Pb mixture isimproved; compared with a commonly used Pb isotope determination methodwith a static multi-collection protocol by the multiple ion counters,the determination method completely eliminates influences of gaindifferences of the different ion counters on the determination resultsof the Pb isotopes, so that an analytical precision is greatly improved.6. The determination method, as recited in claim 1, wherein: thedetermination method is especially applicable to Pb isotope analysis fora single-grain zircon ID-TIMS U—Pb dating method.
 7. The determinationmethod, as recited in claim 1, wherein: the protocol of the establisheddetermination method has universality and is suitable for various typesof mass spectrometers equipped with a multi-ion counter system which canbe applied for Pb isotope analysis of a ²⁰⁵Pb—Pb mixture.